1. Field of the Invention
This invention relates to a method and apparatus for automatic focussing that measures the distance between an object and an objective lens for carrying out focussing based on the measured value.
2. Description of Related Art
In the field of optical instruments, such as optical microscopes or depth meters, an automatic focussing device has hitherto been used for automatic focussing of an object being observed.
Among the automatic focussing devices, there are those in which a laser light beam is emitted to an object and the laser light beam reflected by the object is detected for measuring the distance from the object in order to effect automatic focussing. This automatic focussing device includes a photodetector, such as a photodiode, and detects the reflected laser light from this photodetector.
If the automatic focussing device is used for an optical microscope, the laser light is emitted from a light source so as to be illuminated via an objective lens on the object. The laser light illuminated via the objective lens is reflected by the object so as to be again illuminated on the photodetector via the objective lens.
The reflected laser light forms a spot on the photodetector in association with the distance between the object and the objective lens. With the optical microscope having the automatic focussing device loaded thereon, the objective lens is moved for forming a spot of the reflected laser light beam at a pre-set position on the photodetector. This achieves focussing on the optical microscope.
Meanwhile, the above-described automatic focussing device has been developed for an optical pickup used for, for example, an optical disc. In such automatic focussing device, the laser light beam whose light spot can be reduced to a circular spot is radiated from the light source.
In such case, attempts have been made for reducing the diameter of the laser light spot in the automatic focussing device to as small a value as possible. In this manner, the automatic focussing device achieves fine focussing for an optical instrument, such as an optical pickup.
With these automatic focussing device, used for an optical instrument, such as an optical microscope, focussing needs to be achieved for an object presenting significant differences between bright and dark portions or having a non-smooth surface state. If the surface state of the object is poor, and a laser light beam having a circular light spot of a small diameter is used by the automatic focussing device, the reflected laser light is disturbed. Thus, with the automatic focussing device, the reflected laser light cannot be detected optimally by the photodetector, thus disabling focussing.
On the other hand, if the automatic focussing device is used for an optical instrument, such as an optical microscope, focussing needs to be realized for a field of view larger than the spot diameter of the laser light beam. However, since the laser light beam having a small spot diameter is illuminated on the object, focussing is only for a point in the field of view of the optical microscope illuminated by the laser light. If focussing is only for a point in the field of view of the optical microscope, there is an inconvenience that focussing cannot be realized for other portions.
For obviating this problem, it has been proposed to scan the circular spot of a reduced diameter across the entire field of view and measured values obtained for respective points on the scanning line are averaged to give a mean value which is used for realizing the focussing. However, if this method is used, it becomes necessary to provide a mechanism for scanning the laser light beam thus complicating the structure and consuming time for focussing.